Part 1 Gravity: A Force of Attraction

Gravity is a force of attraction between objects. We're not talking about finding someone really cute and adorable. We mean when the molecules of one object pull on the molecules of another object. It's like the Earth pulling on you and keeping you on the ground. That pull is gravity at work.

All matter is affected by gravity Do you remember the definition of matter? Matter is anything that has mass and volume. Since all matter has mass, all matter is affected by gravity. Gravity (aka: gravitational force) pulls objects towards each other. It acts on anything with mass.

Ok, then why don’t we see objects being pulled towards one another? This is because the mass of most objects is too small to cause an attraction large enough to cause the objects to move towards each other. Even though gravity is “pulling” the pencil you’re holding, its mass is so small that it’s not really moving. There is, however, one object that is big enough to cause a noticeable attraction…

Gravity on Earth That’s right, the Earth! Earth has an enormous mass and thus an enormous gravitational force. When the Earth spins and gravity pulls on the clouds, weather can be affected. The Earth's gravity even holds the atmosphere close to our surface.

The Law of Universal Gravitation Now that we know what gravity is, let’s go back to Newton. He generalized his observation in something called the Law of Universal Gravitation. This law states: All objects in the universe attract each other through gravitational force.

The Law of Universal Gravitation The size of the gravitational force depends on two things: Mass of the objects Distance between the objects

The law of universal gravitation The formula for this law is: F = G x m1 x m2 r2 F = force G = gravitational constant = 6.673 x 10-11 Nm2/kg2 - always an uppercase G, do not confuse with g, which is for gravity M = objects’ mass R = distance between objects

The law of universal gravitation Basically, there are two major parts to this law that you need to know: Gravitational force increases as mass increases Gravitational force decreases as distance increases

What is weight? Weight is a measure of the gravitational force exerted on an object. Most of the time, when we’re talking about weight, we’re referring to the Earth’s gravitational force on an object. Since gravity is a force and weight is a measure of gravity, weight is expressed in newtons (N). On Earth, a 100 gram object would weigh 1 N.

What is mass? Mass is the amount of matter in an object. This does not change… ever! Whereas weight changes when gravity changes, mass always remains the same. Remember, mass is measured with a balance, where the mass of one object is compared to another object. On Earth, mass and weight are both constant since gravity is a constant force, which is why they seem like the same thing to us.

The role of gravity in orbit In space, there is very little gravity. This is because the distances between you and other large objects are so vast, the force is minimal. You would still have mass, and because there is still a little bit of gravity, you would still have some weight.

The role of gravity in orbit We know that an object in orbit is traveling in a circular or near-circular path around another object. There are two forces that act on a spaceship in orbit. First, gravity is pulling the spaceship back to earth, so technically, the shuttle is in free fall. Second, the shuttle is being propelled forward and is moving at a constant speed. As a result, the path of the orbit is a curved combination of the two forces.

UNIVERSAL GRAVITATION acceleration due to gravity, g = Gme/r2 Also known as the gravitational field strength me: earth's mass (5.97×1024 kg) r: distance from earth’s center (6.38×106 m + altitude) g is only 9.80 m/s2 at sea level – it decreases as altitude increases g is different on other planets & moons (it depends on the planet’s mass and radius)

UNIVERSAL GRAVITATION Orbits: gravity provides the centripetal force stable orbit: Fc = Fg orbit speed v = √Gme/r orbit period T = 2pr/v geosynchronous orbit: T = 24.0 hrs, satellite stays over same position on earth

UNIVERSAL GRAVITATION Orbits always falling but never reaching the ground "Weightlessness" is NOT gravity-less no gravity = no orbit weightless is no normal force